The ST-4 cable is a RJ-12 6p6c straight through wiring.
See the Shoestring notes:
http://www.store.shoestringastronomy...ort_cables.pdf
When this is connected between the camera and the mount, PHD should be able to send control signals to the mount.
In PHD you can use the manual guide buttons to see if the mount is reacting to the input signal.
Do don't say what guide scope you have?